Laser welding method

ABSTRACT

A laser welding method is provided to ensure a sufficient joining strength between metal plates by increasing the area of a joining region while preventing “burn through” of a molten metal. In the laser welding method by applying a laser beam to a surface of multiple metal plates superimposed on each other, a scanning locus with the laser beam is sequentially shifted from an inner circular scanning locus to an outer one in a predetermined joining region on the metal plates, and an emission interval is provided to temporally stop the metal-plate-surface irradiation when the scanning locus is shifted. Thus, every time the scanning locus is shifted, the molten metal due to the previous irradiation is cooled and increases its viscosity. Accordingly, the “burn through” is prevented regardless of increase of the area of the joining region, which results in a sufficient joining strength between the metal plates.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119(a) toJapanese Patent Application No. 2018-83609, filed on Apr. 25, 2018. Thecontents of this application are incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present invention relates to a laser welding method for joining aplurality of metal plates superimposed on each other. Particularly, thepresent invention relates to improvement of an irradiation mode on ametal plate with a laser beam.

BACKGROUND ART

Conventionally, laser welding is known as a method for joining (welding)a plurality of metal plates superimposed on each other. Patent Document1 discloses a method for welding two metal plates superimposed on eachother, which includes the steps of: irradiating a surface of the metalplates with a laser beam; melting the metal plates to form a moltenpool; and irradiating the molten pool with the laser beam to flow moltenmetal in the molten pool. Thus, the joining strength between the metalplates is ensured.

PRIOR ART DOCUMENT

Patent Document

-   Patent Document 1: JP 2012-228715 A

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

However, when a gap formed between the metal plates (i.e. a plate gap)is large, it is necessary to increase the amount of molten metal inorder to make the molten metal bridge the gap over the respective metalplates. In this case, a welding defect called as “burn through” may begenerated. That is, the molten metal of the metal plate irradiated withthe laser beam (i.e. an upper plate when the metal plates aresuperimposed on each other in the vertical direction) may fall towardthe other metal plate (i.e. a lower plate when the metal plates aresuperimposed on each other in the vertical direction). In particular,when an area of the joining region (i.e. an area of the molten metalthat hardens in plan view of the metal plate) on the metal plate isincreased in order to ensure a sufficient joining strength between themetal plates, a larger amount of molten metal is needed. Accordingly,the “burn through” is likely to occur, which results in an insufficientjoining strength.

FIG. 15 are diagrams illustrating a state of metal plates having anincreased joining region during irradiation with the laser beam in theconventional laser welding. FIG. 15(a) is a cross-sectional view of thejoining region of the metal plates and the laser beam. FIG. 15(b) is aplan view of the metal plates. In FIG. 15 , the laser welding isperformed in a state in which the gap formed between the metal plates(an upper plate a and a lower plate b) is large, where the laser beam isindicated by the dashed dotted line. A molten metal c of the upper platea hangs down toward the lower plate b. When the irradiation with thelaser beam is continued under the circumstances, the molten metal c maybe detached from the upper plate a to generate the “burn through”, asshown in FIG. 16 (diagrams illustrating a state in which the irradiationwith the laser beam is completed, which respectively correspond to thediagrams of FIG. 15 ). That is, in FIG. 16 , the molten metal c is notbridged between the upper plate a and the lower plate b at the partenclosed by the circle indicated by the dashed double-dotted line, whichmeans that a sufficient joining strength cannot be ensured.

As described above, in the conventional art, it is difficult tosufficiently obtain the joining strength between the metal plates byincreasing the area of the joining region while preventing the “burnthrough” of the molten metal.

The present invention was made in consideration of the above problem, anobject of which is to provide a laser welding method capable of ensuringa sufficient joining strength between metal plates by increasing an areaof a joining region while preventing the “burn through” of a moltenmetal.

Means for Solving the Problem

As a means for solving the above problem to achieve the object of thepresent invention, a laser welding method for joining metal plates isprovided, which includes the steps of: applying a laser beam to asurface of a plurality of metal plates superimposed on each other;melting the metal plates by scanning a position to be irradiated so asto make a molten pool constituted of the molten metal; stirring themolten pool by scanning with the laser beam; shifting sequentially ascanning locus to be scanned with the laser beam from an inner circularscanning locus to an outer circular scanning locus in a predeterminedjoining region on the metal plates; and providing an emission intervalat the time of shifting the scanning locus to be scanned with the laserbeam so that the irradiation on the surface of the metal plates with thelaser beam is temporally stopped.

With the above-described configuration, when a plurality of metal platessuperimposed on each other is joined by laser welding, the scanning withthe laser beam (applying the laser beam) is performed along the innercircular scanning locus in the predetermined joining region on the metalplates so as to melt the metal (metal material) in the vicinity of thescanning locus. A molten pool constituted of the molten metal is scannedand stirred with the laser beam. After that, the irradiation on thesurface of the metal plates with the laser beam is temporally stoppedwhen the scanning locus to be scanned with the laser beam is shifted tothe outer circular scanning locus. In other words, after a certainemission interval (i.e. period for which the irradiation on the surfaceof the metal plates with the laser beam is temporally stopped) elapses,the scanning with the laser beam (applying the laser beam) is re-startedalong the outer circular scanning locus so as to melt the metal materialin the vicinity of the outer circular scanning locus. A molten poolconstituted of the molten metal is scanned and stirred with the laserbeam. By repeatedly preforming the above procedures, every time thescanning locus to be scanned with the laser beam is shifted, a period isprovided to cool the metal that has been molten due to the previousirradiation with the laser beam. Such lowering of the temperature of themolten metal increases its viscosity, which leads to reduction in themass of the metal molten by irradiation with the laser beam after thescanning locus to be scanned with the laser beam is shifted (i.e. themass of the molten metal having the low viscosity is reduced). As aresult, it is possible to prevent generation of “burn through” of themolten metal. That is, the “burn through” can be reduced even when thearea of the joining region is increased. Therefore, it is possible toensure the sufficient joining strength between the metal plates byincreasing the area of the joining region while preventing the “burnthrough” of the molten metal.

It is preferable that the circular scanning loci are concentric circlescentered at a central part of the joining region.

In this case, the scanning with the laser beam is successively performedalong the respective concentric circular scanning loci, and theirradiation on the surface of the metal plates with the laser beam istemporally stopped at the time of shifting the scanning locus. In thisway, it is possible to realize the circular joining region having alarge area while preventing the “burn through” of the molten metal.

When each of all the emissions of the laser beam is performed along thecorresponding circular scanning locus at the emission interval, it ispreferable that the output of the laser beam that is applied to theinnermost circular scanning locus in the joining region is set largerthan the output of the laser beam that is applied to any of the othercircular scanning loci.

In the case where each of all the emissions of the laser beam isperformed along the corresponding circular scanning locus at theemission interval, when the laser beam is applied to the innermostcircular scanning locus in the joining region, the metal material of thejoining region has not at all been molten yet. Thus, the metal materialrequires a large heat input in order to be molten compared to the casein which the laser beam is applied to any of the other scanning loci(i.e. the case in which the metal material has already been molten atthe inner circular scanning locus). Taking into account the abovecircumstances, in this means for solving the problem, the output of thelaser beam applied to the innermost circular scanning locus is setlarger than the output of the laser beam applied to any of the othercircular scanning loci, so that the metal material in the vicinity ofthe innermost circular scanning locus is effectively molten and the timerequired to perform the laser welding is shorten.

Also, when the laser beam is applied to one point of the central part ofthe joining region before application of the laser beam to the innermostcircular scanning locus in the joining region, it is preferable that theoutput of the laser beam applied to the one point of the central part isset larger than the output of the laser beam that is applied to any ofthe other circular scanning loci.

In the case where the laser beam is applied to one point of the centralpart of the joining region before application of the laser beam to theinnermost circular scanning locus in the joining region, when the laserbeam is applied to the one point of the central part of the joiningregion, the metal material of the joining region has not at all beenmolten yet. Thus, the metal material requires a large heat input inorder to be molten. Taking into account the above circumstances, in thismeans for solving the problem, the output of the laser beam applied tothe one point of the central part of the joining region is set largerthan the output of the laser beam applied to the circular scanning loci,so that the metal material in the central part of the joining region iseffectively molten and the time required to perform the laser welding isshorten.

It is preferable that the length of the emission interval is set longeras the circular scanning locus to be scanned with the laser beam isshifted to the outer circular scanning locus.

When the scanning locus to be scanned with the laser beam issequentially shifted from the inner circular scanning locus to the outercircular scanning locus in the joining region, the subsequentirradiation with the laser beam (i.e. application of the laser beamalong the outer circular scanning locus) is performed before the moltenmetal has completely hardened, even when the emission interval isprovided. In other words, the subsequent irradiation with the laser beamis performed on the metal plates in which heat is stored. The amount ofheat stored in the metal plates has a tendency to increase as theirradiation with the laser beam is shifted between the adjacent two ofthe circular scanning loci on the outer side. Taking into account theabove circumstances, in this means for solving the problem, the lengthof the emission interval is set longer as the circular scanning locus tobe scanned with the laser beam is shifted to the outer one, so thathardening of the molten metal is accelerated (in other words, theviscosity is reduced). Therefore, the “burn through” of the molten metalis reliably prevented.

Advantageous Effect of the Invention

In the laser welding in which the laser beam is applied to a surface ofa plurality of metal plates superimposed on each other of the presentinvention, the scanning locus to be scanned with the laser beam issequentially shifted from an inner circular scanning locus to an outercircular scanning locus in a predetermined joining region on the metalplates, and an emission interval is provided at the time of shifting thescanning locus to be scanned with the laser beam so that the irradiationon the surface of the metal plates with the laser beam is temporallystopped. In this way, every time the scanning locus to be scanned withthe laser beam is shifted, a period is provided to cool the metal thathas been molten due to the previous irradiation with the laser beam, andsuch lowering of the temperature of the molten metal increases itsviscosity. As a result, the “burn through” of the molten metal can bereduced even when the area of the joining region is increased.Therefore, it is possible to ensure the sufficient joining strengthbetween the metal plates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram illustrating a laser weldingapparatus used for laser welding according to an embodiment.

FIG. 2 are diagrams illustrating a first laser beam emitting step. FIG.2(a) is a plan view illustrating a metal plate and a scanning locus ofthe laser beam. FIG. 2(b) is a cross-sectional view illustrating ajoining region between metal plates and the laser beam.

FIG. 3 are diagrams respectively corresponding to FIG. 2 in a secondlaser beam emitting step.

FIG. 4 are diagrams respectively corresponding to FIG. 2 in a thirdlaser beam emitting step.

FIG. 5 are diagrams respectively corresponding to FIG. 2 in a fourthlaser beam emitting step.

FIG. 6 are diagrams respectively corresponding to FIG. 2 in a fifthlaser beam emitting step.

FIG. 7 is a graph indicating change in laser output in the laser beamemitting steps.

FIG. 8 is a cross-sectional view illustrating the joining region of themetal plates as an experimental result when two metal plates are weldedby the laser welding method according to the embodiment.

FIG. 9 is a cross-sectional view illustrating the joining region of themetal plates as an experimental result when two metal plates are weldedby the conventional laser welding method.

FIG. 10 is a cross-sectional view illustrating the joining region of themetal plates as an experimental result when three metal plates arewelded by the laser welding method according to the embodiment.

FIG. 11 is a cross-sectional view illustrating the joining region of themetal plates as an experimental result when three metal plates arewelded by the conventional laser welding method.

FIG. 12 is a graph indicating change in laser output in the laser beamemitting steps in Variation 1.

FIG. 13 is a graph indicating change in laser output in the laser beamemitting steps in Variation 2.

FIG. 14 are plan views illustrating the metal plate having a pluralityof joining regions. FIG. 14(a) is a plan view illustrating the metalplate having two joining regions adjacent to each other. FIG. 14(b) is aplan view illustrating the metal plate having three joining regionsadjacent to one another.

FIG. 15 are diagrams illustrating a state of the metal plates duringirradiation with the laser beam in the conventional art. FIG. 15(a) is across-sectional view illustrating the joining region of the metal platesand the laser beam. FIG. 15(b) is a plan view illustrating the metalplate.

FIG. 16 are diagrams respectively corresponding to FIG. 15 , whichillustrate a state in which the irradiation with the laser beam iscompleted in the conventional art.

DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings. In this embodiment, a description willbe given on a case in which the present invention is applied as a laserwelding method that is performed by a laser welding apparatus used in amanufacturing process of vehicles bodies.

—Schematic Configuration of Laser Welding Apparatus—

FIG. 1 is a schematic configuration diagram illustrating a laser weldingapparatus 1 used for laser welding according to this embodiment. Asshown in FIG. 1 , the laser welding apparatus 1 includes a laseroscillator 2, a laser scanner 3, a welding robot 4 and a robotcontroller 5.

The laser oscillator 2 generates a laser beam. The generated laser beamis guided to the laser scanner 3 via an optical fiber cable 21. It ispossible to use the laser beam such as a carbon dioxide laser, a YAGlaser, or a fiber laser.

The laser scanner 3 irradiates a workpiece W, which is formed bysuperimposing two plate materials made of aluminum alloy (aluminum metalplates, hereinafter occasionally referred to as simply “metal plates”)W1 and W2 on each other, with the laser beam guided via the opticalfiber cable 21 (see the dashed dotted line in FIG. 1 ). In the laserscanner 3, a plurality of lenses (not shown) and a plurality of mirrors31 (in FIG. 1 , only one mirror 31 is shown) are housed. As the lenses,the laser scanner 3 includes, for example, a collimating lens to convertthe laser beam into a parallel beam and a condensing lens to convergethe laser beam such that the converged laser beam is focused on amachining point of the workpiece W (i.e. on a predetermined positionirradiated with the laser beam on the workpiece W). Also, each mirror 31is rotatable about a rotary shaft 32. Specifically, the rotary shaft 32is coupled to a scanning motor 33, and the scanning motor 33 isactivated to rotate the rotary shaft 32 that causes the mirror 31 torotate. Then, the workpiece W is scanned with the laser beam by therotation of the mirror 31, and thus the position irradiated with thelaser beam can be changed within the predetermined area on the workpieceW. In this way, it is possible to change the position irradiated withthe laser beam without moving the laser scanner 3 itself. Each of themirrors 31 may, for example, be constituted of a galvanometer mirror.

The laser welding according to this embodiment is a so-called laserscrew welding (LSW). That is, a welding part (a joining region) of theworkpiece W is scanned with the laser beam along a predeterminedscanning locus so that the welding part is molten to be welded. Thescanning with the laser beam is performed by the mirrors 31. Thescanning with the laser beam will be described in detail later.

The welding robot 4 is configured to cause the laser scanner 3 to move.The welding robot 4 is constituted of an articulated robot.Specifically, the welding robot 4 of this embodiment includes: a base41; a rotation mechanism (not shown) housed in the base 41; joints 42,43 and 44; and arms 45, 46 and 47. The laser scanner 3 can be moved in adesired direction by rotating motions of the rotation mechanism andswinging motions of the arms 45, 46 and 47 about the joints 42, 43 and44.

Information for moving the laser scanner 3 toward a part to be welded(for example, information on respective amounts of rotation angles atthe joints 42, 43 and 44) is stored in the robot controller 5 byoff-line teaching previously performed. When a vehicle body is conveyedto the welding work station on the vehicle body manufacturing line, thewelding robot 4 operates based on the above information in response to acontrol signal from the robot controller 5. Thus, the laser scanner 3 isfaced to the part to be welded so that the laser beam is emitted fromthe laser scanner 3 to the part to be welded. In this way, the laserwelding is sequentially performed.

Also, the robot controller 5 includes a laser scanning control unit 51to output a control signal to change the position to be irradiated withthe laser beam on the workpiece W. The laser scanning control unit 51outputs the control signal to the scanning motor 33. The scanning motor33 operates in response to the control signal so as to perform thescanning with the laser beam by rotation of the respective mirrors 31about the rotary shafts 32. Thus, the position to be irradiated with thelaser beam on the workpiece W is moved. The movement of the position tobe irradiated with the laser beam (i.e. the scanning) on the workpiece Wwill be described later.

—Welding Method—

Here, a welding method (laser welding method) as a characteristicfeature of this embodiment is described. In this embodiment, adescription is given on the case in which two metal plates W1 and W2superimposed on each other in the vertical direction are subjected to alap welding. In particular, the superimposed part of the metal plates W1and W2 is irradiated, from the above, with the laser beam emitted by thelaser scanner 3. Hereinafter, the upper metal plate is referred to as anupper plate W1 while the lower metal plate is referred to as a lowerplate W2.

The laser welding according to this embodiment is performed by scanninga joining region previously determined on the upper plate W1 with thelaser beam. Specifically, a plurality of (in this embodiment, five)concentric circle shaped scanning loci (annular scanning loci; i.e.circular scanning loci in the present invention) centered at a centralpart of the joining region is previously determined. Then, the scanninglocus to be scanned with the laser beam is sequentially shifted from theinner circular scanning locus to the outer circular scanning locus outof the circular scanning loci. The joining region is set as a circularregion having a predetermined outer diameter size. The outer diametersize is previously determined as a size to obtain a desirable joiningstrength between the metal plates W1 and W2, taking into account, forexample, the rigidity of a vehicle.

The respective circular scanning loci are five concentric circlescentered at the central part of the joining region. The scanning withthe laser beam is performed sequentially from a first scanning locus SC1defined as the innermost locus (see FIG. 2 ) to a fifth scanning locusSC5 defined as the outermost locus (see FIG. 6 ). When the scanninglocus to be scanned with the laser beam is shifted, an emission intervalis provided so as to temporally stop the emission of the laser beam tothe surface of the upper plate W1. By providing this emission interval,a period for cooling the metal that has been molten due to irradiationwith the laser beam can be ensured, which results in a high viscosity ofthe molten metal due to lowering of the temperature thereof. Also byproviding the emission interval, the subsequent emission of the laserbeam is performed (i.e. the laser beam is applied along the outercircular scanning locus) after the temperature of the molten metal hasdecreased. Thus, it is possible to reduce the heat input per unit volumeto the molten metal and to reduce spatter (i.e. scattering of the moltenmetal). As a result, it is possible to prevent thinning of the moltenmetal that is bridged over the upper plate W1 and the lower plate W2.

As an example of operations to temporally stop the emission of the laserbeam to the surface of the upper plate W1, the output of the laser beamfrom the laser oscillator 2 can be temporally stopped. Also, the focalposition of the laser beam that is applied along each scanning locus SC1to SC5 may be a position where the laser beam penetrates the upper plateW1 or a position where the laser beam does not penetrate the upper plateW1 (i.e. a position on the surface of the upper plate W1).

Hereinafter, the laser beam emitting steps will be specificallydescribed. FIGS. 2 to 6 show the respective laser beam emitting steps.Among them, the respective Figures (a) each show a plan view of theworkpiece W and the scanning locus of the laser beam while therespective Figures (b) each show a cross-sectional view illustrating thejoining region of the workpiece W and the laser beam (more specifically,each state in which the scanning position with the laser beam is movedat the rightmost position and at the leftmost position of the Figure).

As shown in these Figures, in the laser welding according to thisembodiment, five stages of emission of the laser beam (scanning with thelaser beam) are performed (i.e. from the first laser beam emitting stepto the fifth laser beam emitting step) such that a joining region havingthe desired outer diameter size is formed.

The scanning locus (concentric circular scanning locus) to be scannedwith the laser beam in this case has the diameter size that graduallyincreases from the first laser beam emitting step to the fifth laserbeam emitting step in this order. Also, the outputs of the laser beam inthe respective laser beam emitting steps have the same value. FIG. 7 isa graph indicating change in the output of the laser beam in the laserbeam emitting steps. In FIG. 7 , the period t1 indicates the period forthe first laser beam emitting step, the period t2 indicates the periodfor the second laser beam emitting step, the period t3 indicates theperiod for the third laser beam emitting step, the period t4 indicatesthe period for the fourth laser beam emitting step, and the period t5indicates the period for the fifth laser beam emitting step. As shown inFIG. 7 , there is a period in which the output of the laser beam iszero, i.e. the emission interval between each adjacent two of the laserbeam emitting steps.

—Respective Laser Beam Emitting Steps—

Hereinafter, the respective laser beam emitting steps are described.

FIG. 2 indicate the first laser beam emitting step (the laser beamemitting step performed for the first time). In the first laser beamemitting step, the scanning locus (the first scanning locus SC1) withthe laser beam has the smallest diameter size (that is, the diametersize is smaller than the respective diameter sizes of the scanning lociSC2 to SC5 with the laser beam in the other laser beam emitting steps).Accordingly, the molten part of the metal material of the upper plate W1and the lower plate W2 also has a small diameter size. In other words,the volume of the molten metal F is small. In the first laser beamemitting step, the laser beam is applied along the first scanning locusSC1 such that the laser beam is rotated (revolved) multiple times alongthe first scanning locus SC1. The number of rotations (the number ofrevolutions) of the laser beam is experimentally determined as a valueto obtain the volume of the molten metal F to be bridged over the upperplate W1 and the lower plate W2.

In the first laser beam emitting step in which the laser beam is rotatedmultiple times along the first scanning locus SC1, the upper plate W1and the lower plate W2 are molten at the first scanning locus SC1 and inthe vicinity thereof. Thus, a molten pool P is formed by the moltenmetal F. By the scanning with the laser beam, the molten metal F in themolten pool P is stirred and flowed. That is, the molten metal F isagitated within the molten pool P. In this case, the molten pool P isformed so as to have a cone shape due to the flow of the molten metal Fin the circumferential direction. At the same time, the molten metal Fis undulated in the molten pool P. The molten pool P in which the moltenmetal F is undulated is aggregated due to the surface tension of themolten metal F, thus is formed as a joining part that does not have ahollow or a separated bead. In this way, the upper plate W1 and thelower plate W2 are integrally welded. As described above, since themolten metal F is undulated while flowing in the molten pool P due tothe scanning with the laser beam, the molten pool P is sufficientlymolten and stirred so that bubbles are satisfactorily discharged. Sincethe molten pool P is also undulated while flowing due to the scanningwith the laser beam, the molten pool P is sufficiently stirred so thatelements are sufficiently diffused to prevent segregation, and that thetemperature is uniformized to reduce the heterogeneous state of thestructure.

FIG. 3 indicate the second laser beam emitting step (the laser beamemitting step performed for the second time). In FIG. 3 , the regionindicated by the hatched lines is the joining region made by hardeningthe metal that has been molten in the first laser beam emitting step.The second laser beam emitting step is performed at the emissioninterval after termination of the first laser beam emitting step. Thediameter size of the scanning locus with the laser beam in the secondlaser beam emitting step (the second scanning locus SC2) is larger thanthe diameter size of the first scanning locus SC1. Accordingly, themolten part of the metal material of the upper plate W1 and the lowerplate W2 in the second laser beam emitting step also has a diameter sizelarger than the diameter size of the molten part of the metal materialof the upper plate W1 and the lower plate W2 in the first laser beamemitting step. In the second laser beam emitting step, the laser beam isapplied along the second scanning locus SC2 such that the laser beam isrotated (revolved) multiple times along the second scanning locus SC2.The number of rotations (the number of revolutions) of the laser beam isexperimentally determined as a certain value to obtain the volume of themolten metal F to be newly bridged over the upper plate W1 and the lowerplate W2.

In the second laser beam emitting step, the upper plate W1 and the lowerplate W2 are molten at the second scanning locus SC2 and in the vicinitythereof, similarly to the above-described first laser beam emittingstep. Thus, the molten pool P is formed by the molten metal F. By thescanning with the laser beam, the molten metal F in the molten pool P isstirred and flowed. That is, the molten metal F is agitated within themolten pool P. Since the molten metal F is undulated while flowing inthe molten pool P due to the scanning with the laser beam, bubbles aresatisfactorily discharged from the molten pool P. Also in the moltenpool P, elements are sufficiently diffused to prevent segregation, andthe temperature is uniformized to reduce the heterogeneous state of thestructure.

FIG. 4 indicate the third laser beam emitting step (the laser beamemitting step performed for the third time). FIG. 5 indicate the fourthlaser beam emitting step (the laser beam emitting step performed for thefourth time). FIG. 6 indicate the fifth laser beam emitting step (thelaser beam emitting step performed for the fifth (final) time). In theseFigures also, each region indicated by the hatched lines is the joiningregion made by hardening the metal that has been molten in the previousstep. Each of these laser beam emitting steps is performed at theemission interval after termination of the previous step. The diametersize of the scanning locus to be scanned with the laser beam in each ofthese laser beam emitting steps (specifically, the third scanning locusSC3, the fourth scanning locus SC4 and the fifth scanning locus SC5) islarger than the diameter size of the scanning locus in the previousstep. Accordingly, the molten part of the metal material of the upperplate W1 and the lower plate W2 in each of the laser beam emitting stepsalso has a diameter size larger than the diameter size of the moltenpart of the metal material of the upper plate W1 and the lower plate W2in the previous step. In each of these laser beam emitting steps, thelaser beam is applied along the corresponding scanning locus to bescanned with the laser beam (specifically, the third scanning locus SC3,the fourth scanning locus SC4 and the fifth scanning locus SC5) suchthat the laser beam is rotated (revolved) multiple times along thecorresponding scanning locus. The number of rotations (the number ofrevolutions) of the laser beam is experimentally determined as a certainvalue to obtain the volume of the molten metal F to be newly bridgedover the upper plate W1 and the lower plate W2.

In the respective steps from the third laser beam emitting step to thefifth laser beam emitting step also, the upper plate W1 and the lowerplate W2 are molten at the respective scanning loci SC3, SC4 and SC5 andin the vicinities thereof, similarly to the above-described first laserbeam emitting step. Thus, the molten pool P is formed by the moltenmetal F. By the scanning with the laser beam, the molten metal F in themolten pool P is stirred and flowed. Thus, bubbles are satisfactorilydischarged from the molten pool P. Also in the molten pool P, elementsare sufficiently diffused to prevent segregation, and the temperature isuniformized to reduce the heterogeneous state of the structure.

Welding conditions for the respective laser beam emitting steps arespecifically described. Examples of the welding conditions include: thediameter size of the scanning locus to be scanned with the laser beam;the scanning speed; the output of the laser beam; and the length of theemission interval. Note that specific values of the welding conditionsare listed hereinafter as an example in which the upper plate W1 and thelower plate W2 both have a plate thickness of 1.0 mm and their plate gapis 0.8 mm.

In this embodiment, the diameter size of the scanning locus to bescanned with the laser beam is set such that the respective diametersizes of the scanning loci SC1 to SC5 increase at regular intervals fromthe first laser beam emitting step to the fifth laser beam emittingstep. For example, the diameter size of the first scanning locus SC1 inthe first laser beam emitting step is 0.4 mm, the diameter size of thesecond scanning locus SC2 in the second laser beam emitting step is 0.8mm, the diameter size of the third scanning locus SC3 in the third laserbeam emitting step is 1.2 mm, the diameter size of the fourth scanninglocus SC4 in the fourth laser beam emitting step is 1.6 mm, and thediameter size of the fifth scanning locus SC5 in the fifth laser beamemitting step is 2.0 mm. However, these values are not limited thereto.

Also, the scanning speed is the same through all of the first laser beamemitting step to the fifth laser beam emitting step. For example, thescanning speed is set to 20 m/min. However, this value also is notlimited thereto. Furthermore, the respective laser beam emitting stepsmay have different scanning speeds.

Also, the output of the laser beam is the same through all of the firstlaser beam emitting step to the fifth laser beam emitting step, asdescribed above. For example, the output of the laser beam is set to4,000 W. However, this value also is not limited thereto.

Also, the respective lengths of the emission intervals that are setbetween the laser beam emitting steps are the same. The length of theemission interval is set as a period of time that is needed to stabilizethe cone-shaped molten pool P made of the metal molten in one laser beamemitting step after termination of this laser beam emitting step. Forexample, the length of the emission interval is set to 0.05 sec.However, this value is not limited thereto.

Under the welding conditions set as described above, the first laserbeam emitting step to the fifth laser beam emitting step aresequentially performed. Thus, the upper plate W1 and the lower plate W2are integrally joined at the joining region.

—Effect Provided by Embodiment—

As described above, in this embodiment, the laser beam emitting steps(i.e. the first laser beam emitting step to the fifth laser beamemitting step with the emission interval being set between each adjacenttwo thereof) are repeatedly performed. Accordingly, every time thescanning locus to be scanned with the laser beam is shifted, a period isprovided to cool the metal that has been molten due to the previousirradiation with the laser beam. Such lowering of the temperature of themolten metal F increases its viscosity, which leads to reduction in themass of the metal molten by irradiation with the laser beam after thescanning locus to be scanned with the laser beam is shifted (i.e. themass of the molten metal F having the low viscosity is reduced). As aresult, it is possible to prevent generation of the welding defect suchas “burn through” in which the molten metal F is detached from the upperplate W1. That is, the “burn through” can be reduced even when the areaof the joining region is increased. Therefore, it is possible to ensurethe sufficient joining strength between the metal plates W1 and W2 byincreasing the area of the joining region while preventing the “burnthrough” of the molten metal F.

Also, by providing the emission interval, the subsequent irradiationwith the laser beam (i.e. application of the laser beam along the outercircular scanning locus) is performed after the temperature of themolten metal F has lowered. Thus, it is possible to reduce the heatinput per unit volume to the molten metal F and to reduce spatter. As aresult, it is possible to prevent thinning of the molten metal F that isbridged over the upper plate W1 and the lower plate W2. In this wayalso, it is possible to prevent detaching of the molten metal F from theupper plate W1 and to ensure the sufficient joining strength between theupper plate W1 and the lower plate W2.

EXAMPLES

Here, a description will be given on examples conducted in order toconfirm the above-described effects.

As a first Example, the joining regions were respectively made bywelding two metal plates using the laser welding method according to theabove-described embodiment and using the conventional laser weldingmethod, so that the respective cross-sections of the joining regionswere compared to each other.

FIG. 8 is a cross-sectional view illustrating the joining region of themetal plates W1 and W2 as the experimental result when the two metalplates W1 and W2 were welded by the laser welding method according tothis embodiment. FIG. 9 is a cross-sectional view illustrating thejoining region of metal plates w1 and w2 as the experimental result whenthe two metal plates w1 and w2 were welded by the conventional laserwelding method.

In the joining region made by the conventional laser welding method asshown in FIG. 9 , the molten metal f was detached from the upper platew1 to generate “burn through”. In the part where the “burn through” wasgenerated, the molten metal f was not bridged between the upper plate w1and the lower plate w2, which means that the sufficient joining strengthwas not ensured.

In contrast to the above, in the joining region made by the laserwelding method according to this embodiment as shown in FIG. 8 , no“burn through” was generated, and the molten metal F was bridged betweenthe upper plate W1 and the lower plate W2 over the entire joiningregion, which means that the sufficient joining strength was ensured.

Especially, in order to obtain the above-described effects, it ispreferable that a region A (i.e. a bridging part of the molten metal F/fto the upper plate W1/w1) in the Figures has a large cross-sectionalarea while a region B (i.e. a recess formed in the lower plate W2/w2; asthe recess increases, the molten metal F/f is more thinned) has a smallcross-sectional area. These cross-sectional areas were measured. Whenthe laser welding method of this embodiment was applied, the resultshowed that the cross-sectional area of the region A was approximatelytwo times the cross-sectional area of the region B. In contrast, whenthe conventional laser welding method was applied, the result showedthat the cross-sectional area of the region A was only about 90% of thecross-sectional area of the region B.

As a second Example, the joining regions were respectively made bywelding three metal plates using the laser welding method according tothe above-described embodiment and using the conventional laser weldingmethod, so that the respective cross-sections of the joining regionswere compared to each other.

FIG. 10 is a cross-sectional view illustrating the joining region of themetal plates W1, W2 and W3 as the experimental result when the threemetal plates W1, W2 and W3 were welded by the laser welding methodaccording to this embodiment. FIG. 11 is a cross-sectional viewillustrating the joining region of metal plates w1, w2 and w3 as theexperimental result when the three metal plates w1, w2 and w3 werewelded by the conventional laser welding method.

In the joining region made by the conventional laser welding method asshown in FIG. 11 , the molten metal f was detached from the upper platew1 to generate “burn through”. In the part where the “burn through” wasgenerated, the molten metal f was not bridged between the upper plate w1and the intermediate plate w3, which means that the sufficient joiningstrength was not ensured.

In contrast to the above, in the joining region made by the laserwelding method according to this embodiment as shown in FIG. 10 , no“burn through” was generated, and the molten metal F was bridged betweenthe upper plate W1 and the intermediate plate W3 over the entire joiningregion, which means that the sufficient joining strength was ensured.

Especially, in order to obtain the above-described effects, it ispreferable that the region A in the Figures has a large cross-sectionalarea while the region B has a small cross-sectional area. Thesecross-sectional areas were measured. When the laser welding method ofthis embodiment was applied, the result showed that the cross-sectionalarea of the region A was approximately two times half thecross-sectional area of the region B. In contrast, when the conventionallaser welding method was applied, the result showed that thecross-sectional area of the region A was only about 90% of thecross-sectional area of the region B.

(Variation 1)

Here, Variation 1 is described. In this Variation, the output of thelaser beam in the respective laser beam emitting steps is changed. Sincethe other configuration and the welding method are the same as those inthe above-described embodiment, only the change in the output of thelaser beam will be described hereinafter.

FIG. 12 is a graph indicating the change in the output of the laser beamin the laser beam emitting steps in this Variation. In FIG. 12 , theperiod t1 indicates the period for the first laser beam emitting step,the period t2 indicates the period for the second laser beam emittingstep, the period t3 indicates the period for the third laser beamemitting step, the period t4 indicates the period for the fourth laserbeam emitting step, and the period t5 indicates the period for the fifthlaser beam emitting step.

In the above-described embodiment, the output of the laser beam is thesame through all the first laser beam emitting step to the fifth laserbeam emitting step (see FIG. 7 ). In contrast, in this Variation, theoutput of the laser beam in the first laser beam emitting step is setlarger than the output of the laser beam in the respective other laserbeam emitting steps (i.e. from the second laser beam emitting step tothe fifth laser beam emitting step). That is, the output of the laserbeam that is applied to the innermost circular scanning locus in thejoining region is set larger than the output of the laser beam that isapplied to the respective other circular scanning loci. The relationshipbetween the output of the laser beam in the first laser beam emittingstep and the output of the laser beam in the respective other laser beamemitting steps (for example, the output ratio of the laser beam) is setbased on experiments and/or simulations. As one example, the output ofthe laser beam in the first laser beam emitting step is set to 20%higher than the output of the laser beam in the respective other laserbeam emitting steps. However, the above value is not limited thereto.

When the laser beam is applied to the innermost first scanning locus SC1in the joining region, the metal material of the joining region has notat all been molten yet. Thus, the metal material requires a large heatinput in order to be molten compared to the case in which the laser beamis applied to any of the other scanning loci SC2 to SC5 (i.e. the casein which the metal material has already been molten at the innercircular scanning locus). Taking into account the above circumstances,in this Variation, the output of the laser beam applied to the innermostfirst scanning locus SC1 is set larger than the output of the laser beamapplied to any of the other scanning loci SC2 to SC5, so that the metalmaterial in the vicinity of the first scanning locus SC1 is effectivelymolten and the time required to perform the laser welding is shorten. Inthis case, it is possible to ensure the sufficient joining strengthbetween the metal plates W1 and W2 by increasing the area of the joiningregion while preventing the “burn through” of the molten metal F, and inaddition, it is possible to reduce the time required to perform thelaser welding (i.e. a circle time) for one joining region.

(Variation 2)

Here, Variation 2 is described. In this Variation also, the output ofthe laser beam in the respective laser beam emitting steps is changed.Since the other configuration and the welding method are the same asthose in the above-described embodiment, only the change in the outputof the laser beam will be described hereinafter.

FIG. 13 is a graph indicating the change in the output of the laser beamin the laser beam emitting steps in this Variation. In FIG. 13 , theperiod t1 indicates the period for the first laser beam emitting step,the period t2 indicates the period for the second laser beam emittingstep, the period t3 indicates the period for the third laser beamemitting step, the period t4 indicates the period for the fourth laserbeam emitting step, and the period t5 indicates the period for the fifthlaser beam emitting step.

In the above-described embodiment, every length of the emission intervalbetween the adjacent two steps of the first laser beam emitting step tothe fifth laser beam emitting step is the same (see FIG. 7 ). On theother hand, in this Variation, the length of the emission intervalbecomes longer as the laser beam emitting steps proceed. That is, thelength of the emission interval is set longer as the circular scanninglocus to be scanned with the laser beam is shifted to the outer one. Theratio of the respective lengths of the emission intervals is set basedon experiments and/or simulations. As one example, the length of theemission interval is set to 20% longer every time the scanning locus tobe scanned with the laser beam is shifted. However, the above value isnot limited thereto.

When the scanning locus to be scanned with the laser beam issequentially shifted from the inner circular scanning locus to the outercircular scanning locus in the joining region, the subsequentirradiation with the laser beam (i.e. application of the laser beamalong the outer circular scanning locus) is performed before the moltenmetal F has completely hardened, even when the emission interval isprovided. In other words, the subsequent irradiation with the laser beamis performed on the metal plates W1 and W2 in which heat is stored. Theamount of heat stored in the metal plates has a tendency to increase asthe irradiation with the laser beam is shifted between the adjacent twoof the circular scanning loci on the outer side. Taking into account theabove circumstances, in this Variation, the length of the emissioninterval is set longer as the circular scanning locus to be scanned withthe laser beam is shifted to the outer one, so that hardening of themolten metal F is accelerated (in other words, the viscosity isreduced). Therefore, the “burn through” of the molten metal F isreliably prevented.

(Variation 3)

Here, Variation 3 is described. In this Variation, the irradiation onthe joining region with the laser beam differs from that in theabove-described embodiment. Since the other configuration and thewelding method are the same as those in the above-described embodiment,only the aspect of the irradiation with the laser beam will be describedhereinafter.

In the above-described embodiment, the laser welding method isconstituted of only the five steps, namely, the first laser beamemitting step to the fifth laser beam emitting step. In this Variation,an initial laser beam emitting step is performed as a step previous tothe first laser beam emitting step, in which the laser beam is appliedto one point of the central part of the joining region. That is, theposition irradiated with the laser beam is fixed as one point that isnot moved to make a scanning locus, thus only the central part of thejoining region of the metal plates W1 and W2 is molten.

When the initial laser beam emitting step is performed, the output ofthe laser beam in this initial laser beam emitting step is set higherthan the output of the laser beam in the respective other laser beamemitting steps (i.e. the first laser beam emitting step to the fifthlaser beam emitting step). The relationship between the output of thelaser beam in the initial laser beam emitting step and the output of thelaser beam in the respective other laser beam emitting steps (forexample, the output ratio of the laser beam) is set based on experimentsand/or simulations. As one example, the output of the laser beam in theinitial laser beam emitting step is set to 20% higher than the output ofthe laser beam in the respective other laser beam emitting steps.However, the above value is not limited thereto.

When the laser beam is applied to one point of the central part of thejoining region in the initial laser beam emitting step, the metalmaterial of the joining region has not at all been molten yet. Thus, themetal material requires a large heat input in order to be molten. Takinginto account the above circumstances, in this Variation, the output ofthe laser beam applied to the one point of the central part of thejoining region is set larger than the output of the laser beam appliedto any of the circular scanning loci, so that the metal material in thecentral part of the joining region is effectively molten and the timerequired to perform the laser welding is shorten. In this case, it ispossible to ensure the sufficient joining strength between the metalplates W1 and W2 by increasing the area of the joining region whilepreventing the “burn through” of the molten metal F, and in addition, itis possible to reduce the time required to perform the laser welding(i.e. a circle time) for one joining region.

(Variation 4)

Here, Variation 4 is described. In this Variation, the joining regionsmade by the laser welding method according to the above-describedembodiment are disposed respectively at positions adjacent to oneanother.

FIG. 14(a) is a plan view illustrating the workpiece W having twojoining regions adjacent to each other. FIG. 14(b) is a plan viewillustrating the workpiece W having three joining regions adjacent toone another. In both cases, the laser welding method similar to themethod in the above-described embodiment is applied to the respectivejoining regions.

Also, any of the laser welding methods according to the above Variationsmay be applied to the respective joining regions.

OTHER EMBODIMENTS

The present invention is not limited to the above-described embodimentand the Variations. It can be appropriately modified and changed withinthe scope of the appended claims and equivalency thereof.

For example, in the above-described embodiment and the Variations, thedescription was given on the case in which the present invention wasapplied as a laser welding method performed by the laser weldingapparatus 1 in the vehicle body manufacturing process. However, thepresent invention can be applied to the laser welding for other members.

Also, in the above-described embodiment and the Variations, thedescription was given on the case in which the present invention wasapplied as a laser welding method for welding two plate materials madeof aluminum to each other. However, the present invention is not limitedthereto. It may be applied as a laser welding method for welding threeor more plate materials to one another. Also, the plate material towhich is applicable the laser welding method of the present invention isnot limited to aluminum. It may be made of iron, magnesium, titanium orcopper. Furthermore, the present invention can be applied to the weldingof dissimilar metals.

In the above-described embodiment and the Variations, the descriptionwas given on the case in which the metal plates W1 and W2 superimposedon each other in the vertical direction were subjected to the lapwelding. In particular, the workpiece W was irradiated with the laserbeam from the above. However, the present invention is not limitedthereto. It may be applied to the lap welding of the metal platessuperimposed on each other in the horizontal direction. That is, thepresent invention can be applied to the case in which the laser beam isapplied to the workpiece in the horizontal direction.

Also in the above-described embodiment and the Variations, the laserbeam emitting steps (the laser beam emitting steps along the circularscanning loci) are constituted of the five steps from the first laserbeam emitting step to the fifth laser beam emitting step. However, thepresent invention is not limited thereto. The laser beam emitting stepsmay be constituted of four or less laser beam emitting steps, or may beconstituted of six or more laser beam emitting steps.

INDUSTRIAL APPLICABILITY

The present invention is suitably applicable to a laser welding methodin which a lap welding with the laser beam is performed to aluminummetal plates.

REFERENCE SIGNS LIST

-   1 Laser welding apparatus-   2 Laser oscillator-   3 Laser scanner-   W Workpiece-   W1 Upper plate (metal plate)-   W2 Lower plate (metal plate)-   F Molten metal-   P Molten pool

What is claimed is:
 1. A laser welding method for joining metal plates,comprising the steps of: applying a laser beam along a first circularscanning locus a plurality of times, to a surface of a plurality ofmetal plates superimposed on each other; melting the metal plates byscanning, with the laser beam, along the first circular scanning locusso as to make a molten pool constituted of the molten metal; stirringthe molten pool by scanning with the laser beam; after the stirring,temporally stopping the laser beam for an emission interval, wherein theemission interval is a period of time that an output of the laser beamis zero until the molten metal hardens into a joining region on themetal plates, wherein during the emission interval irradiation on thesurface of the plurality of metal plates is stopped; after the emissioninterval, which ends before the molten metal has completely hardened,thereby subsequent irradiation is performed with the laser beam on themetal plates in which heat is stored, applying the laser beam along asecond circular scanning locus a plurality of times, wherein the secondcircular scanning locus is located circumferentially outside the firstcircular scanning locus and outside an outermost circumference of thejoining region that was made during the emission interval by thehardened molten metal, wherein the second circular scanning locus thatis a scanning locus of the laser beam emitted for a final time islocated circumferentially outside of at least the first circularscanning locus that is a scanning locus of the laser beam emitted beforethe final time; melting the metal plates by scanning, with the laserbeam, along the second scanning locus so as to make another molten poolconstituted of the molten metal; after scanning along the secondcircular scanning locus, stirring the another molten pool by scanningwith the laser beam; after the stirring, temporally stopping the laserbeam for the emission interval, so as to increase the joining regionmade by the molten metal hardening; and wherein, an output of the laserbeam that is applied to the first circular scanning locus in the joiningregion is set larger than an output of the laser beam that is applied toany other circular scanning loci.
 2. The laser welding method accordingto claim 1, wherein the first circular scanning locus and the circularsecond scanning locus are concentric circles centered at a central partof the joining region.
 3. The laser welding method according to claim 1,wherein, when the laser beam is applied to one point of a central partof the joining region before application of the laser beam to aninnermost circular scanning locus in the joining region, an output ofthe laser beam applied to the one point of the central part is setlarger than an output of the laser beam that is applied to any of theother circular scanning loci.
 4. The laser welding method according toclaim 1, wherein a length of the emission interval is set longer afterscanning along the second circular scanning locus than after scanningalong the first circular scanning locus.